Examinations of the spatial frequency space find widespread use in many technological fields. Since momentum spaces correspond to spatial frequency spaces, the term spatial frequency space also includes momentum spaces. The designation spatial frequency space serves to clarify that the invention also relates to methods in which no momentum transfer takes place.
A known problem in recoding spatial frequency spaces is that, when a high spatial resolution is combined with a high spatial frequency resolution, a very long measuring time is needed.
The keyhole process is a known procedure for solving this problem. In this process, a high-resolution image is acquired by acquiring the entire spatial frequency space at least at one point in time. In one or more further measuring steps, a central area of the spatial frequency space is recorded, which determines the contrast of the reconstructed image. Subsequently, the high-resolution image is joined mathematically to the image or images recorded of the central areas of the spatial frequency space in such a way that, even for another point in time or for other points in time, an image having a high-resolution effect is acquired with a contrast that corresponds to the point in time of the recording.
This known method entails the drawback that contrast changes between consecutive measurements can only be ascertained if they have a sufficiently large spatial extension.
This drawback is especially detrimental when functional parameters are to be acquired.
Thus, for example, in functional magnetic resonance imaging, there is a need for parameters that influence nuclear magnetic resonance signals to be acquired with the highest possible spatial resolution
The invention relates especially to an imaging process for examining substances in which, by means of indirect nuclear spin interaction, a precession of nuclear spins with an additional phase angle relative to an already present precession is generated in an external magnetic field so that a transversal magnetization fans out perpendicular to the external magnetic field and so that a relaxation of the transversal magnetization with a relaxation time T2 is generated.
Magnetic resonance imaging allows the various physiological parameters to be ascertained. Examples of this are the acquisition of regional cerebral blood volumes (rCBV) and regional cerebral blood flow (rCBF).
With known perfusion experiments, examinations are carried out subsequent to an intravenous injection of a contrast agent, for example, a gadolinium chelate such as Gd-DTPA or Gd-BOPTA.